1. Field of the Invention
The present invention relates to a communications system, and in particular, though not exclusively, to a communication system incorporating a satellite link.
2. Related Art
In communications systems such as those having a satellite link, data is sent in a synchronous block mode to keep down the overhead of control and signalling information. Such a synchronous link is maintained in the absence of user data by transmitting a null character e.g. ASCII value 00H. A problem arises because the user data may contain the null character, and unless action is taken the receiving part of the synchronous link will discard a null in the user data as if it were not data. To indicate such characters to the receiving part of the link it is known to send "Escape" control sequences of characters but these increase the control overhead of a link sharply and are liable to cause significant errors if one of the characters of a sequence is lost in transmission. This is a particular example of the more general problem of transmitting data, over whatever medium, when that data includes so called reserved characters or characters which in the circumstances the receiving part of a link will not recognise as proper data.